The iron carbonate siderite [FeCO3] is a well-known mineral that contains about 48% iron. In fact, the name siderite is derived from the Greek sideros (iron). Although the iron percentage is rather high, siderite is not an important ore since it is tough to smelt, and the ore bodies are typically small and discontinuous. Interestingly, siderite is found as crystals in metamorphic rocks, as a gangue mineral in hydrothermal rocks, in a variety of sedimentary rocks, and in igneous pegmatites.
Siderite and pyrite, Eagle Mine, Gilman District. Width ~ 4.6 cm.
Interestingly, mineralogists have co-opted the name siderite (sideros) for minerals that are not even related to the mineral siderite but do contain iron. This naming has confused rockhounds since one tends to think that a mineral like phosphosiderite is related to, or allied with, siderite. However, this is not the case since phosphosiderite is a hydrated iron phosphate [FePO4-2H2O] that is only related to siderite by the presence of iron.
Photomicrograph of lilac-colored elongated crystals of phosphosiderite collected from the Bull Moose Mine, Black Hills, South Dakota. Width of specimen at bottom of photo is ~ 1.1 cm.
For me the confusing minerals containing the word siderite are the iron arsenates lumped together in the Pharmacosideite Group: one or two cations (iron is always one) + the arsenate anion (AsO4) + hydroxyl ion + water. In the namesake mineral, pharmacosiderite, the two cations are potassium and iron— KFe34(AsO4)3(OH)2-6/7H2O. The “pharma” is derived from the Greek word (φάρμăκου) pharmakou for poison or drug alluding to the presence of arsenic. Other members of the Group have as their major cations: barium, cesium, sodium, lead, strontium, thallium, and a strange water H3O (Hydronium).
Pharmacosiderite crystals have a variety of colors--Olive to emerald green, reddish brown to yellowish brown, different shades of red, yellow. The luster ranges from adamatine to greasy and often is tough to accurately place on such tiny crystals---at least with my camera equipment. However, there are some locations where the crystals are much larger. The crystals are brittle with a white streak and are soft (~2.5 Mohs). Pharmacosiderite is a secondary mineral usually forming in the oxidation zones of ore deposits containing iron and arsenic.
The Gold Hill Mine (Clifton Mining District) in the Deep Creek Mountains in far western Utah was producing mine of As-Pb-Au-Cu-Ag-Zn-W-Ba until 1945. Since then the mine has been a major producer of collectable mineral specimens, especially secondary copper minerals and arsenates; many are micromounts..
I have four specimens from Gold Hill that display very tiny crystals of the Pharmacosiderite Group. Although they often are identified as pharmacosiderite, the potassium-iron arsenate, MinDat lists the mineral as bariopharmacosiderire [Ba.05Fe4(AsO4)3(OH)4-5H2O], an arsenate with barium replacing the potassium. The crystals are generally translucent with a vitreous luster and a variety of colors—brown, green, yellow, yellow-brown, reddish brown, and sometimes bluish. Now, here is the confusing part for me: the crystals in my specimens appear as tiny yellowish cubes and I would assume they belong to the Isometric Crystal System (Cubic); however, Mindat notes they are Tetragonal, and the crystals are pseudocubic. Without specific collecting data, my guess on any of these microcrystals would be assignment to the Pharmacosiderite Group.
Unfortunately, the specimens display extremely tiny crystals (cubes) that are much less than 1 mm. in size and essentially beyond my photographic capabilities. But, I learned much from this little experience.
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